Bipolar Transistor, full name bipolar junction transistor(BJT), is an electronic device with three terminals, made of three parts of semiconductors with different levels of doping. The charge flow in the transistor is mainly due to the diffusion and drift movement of carriers at the PN junction.

I Bipolar and Unipolar Transistor

The bipolar transistor is a revolutionary invention in the history of electronics. Its inventors William Shockley, John Barding, and Walter Braton were awarded the Nobel Prize in Physics in 1956.

The work of this kind of transistor involves the flow of both electron and hole carriers, so it is bipolar and called bipolar carrier transistor. This mode of operation is different from unipolar transistors such as field-effect transistors, which only involve the drift of a single type of carrier. The boundary between the two different dopant accumulation regions is formed by a PN junction.

Bipolar transistors are made of three parts of semiconductors with different levels of doping. The charge flow in the transistor is mainly due to the diffusion and drift movement of carriers at the PN junction. Taking the NPN transistor as an example, according to the design, electrons in the highly doped emitter region move to the base through diffusion. In the base region, holes are majority carriers and electrons are minority carriers. Because the base area is very thin, these electrons reach the collector through drifting motion, thereby forming a collector current, so bipolar transistors are classified as minority carrier devices.

Bipolar transistors can amplify signals and have a good power control, high-speed operation ability, and durability, so they are often used to form amplifier circuits, or drive speakers, motors, and other equipment, and are widely used in aerospace engineering, medical equipment, and robots.

II How does a Bipolar Transistor Work?

Here, we take NPN bipolar transistor as the target to discuss the working principle of bipolar transistors.

The NPN type bipolar transistor can be regarded as two diodes with a common anode joined together. In the normal bipolar transistor operation, the base-emitter junction (the "collector junction") is in a forward-biased state, while the base-collector (the "collecting junction") is in a reverse-biased state.

Figure 1. PNP Bipolar Transistor Cross-section Diagram

When there is no applied voltage, the electron concentration in the N region of the emitter junction (the majority of carriers in this region) is greater than the electron concentration in the P region, and part of the electrons will diffuse to the P region. In the same way, part of the holes in the P area will also diffuse to the N area. So a space charge region (also known as a depletion layer) will be formed on the emitter junction, generating an internal electric field whose direction is from the N region to the P region. This electric field will hinder the further occurrence of the aforementioned diffusion process and achieve a dynamic balance.

At this time, if a forward voltage is applied to the emitter junction, the dynamic balance between the aforementioned carrier diffusion and the internal electric field in the depletion layer will be broken, which will cause thermally excited electrons to be injected into the base region. In an NPN transistor, the base region is P-type doped, where holes are the majority dopants, so electrons in this region are called "minority carriers".

On the one hand, the electrons injected from the emitter into the base region recombine with the majority carrier holes here, on the other hand, because the base region is low doped with thin physical size, and the collector junction is in the reverse bias state, most of the electrons will reach the collector area through drifting motion, forming a collector current.

In order to minimize the recombination of electrons before they reach the collector junction, the base region of the transistor must be made thin enough so that the time required for carrier diffusion is shorter than the lifetime of semiconductor minority carriers.

At the same time, the base thickness must be much smaller than the diffusion length of the electrons (see Fick's law). In modern bipolar transistors, the thickness of the base region is typically a few tenths of microns.

It should be noted that although the collector and emitter are both N-type doping, the degree of doping and the physical properties of the two are not the same. Therefore, the bipolar transistor must be distinguished from the form of two diodes in series in opposite directions.                                                                                           

III Types of Bipolar Transistors

A bipolar transistor consists of three different doped semiconductor regions, which are the emitter region, the base region, and the collector region. These regions are N-type, P-type, and N-type semiconductors in NPN-type transistors, and P-type, N-type, and P-type semiconductors in PNP-type transistors. Each semiconductor area has a pin end, usually with the letters E, B, and C to indicate the emitter, base, and collector.

The physical location of the base is between the emitter and the collector, and it is made of lightly doped, high-resistivity materials. The collector surrounds the base area. Due to the reverse bias of the collector junction, it is difficult for electrons to be injected into the base area from here. This causes the common-base current gain to be approximately equal to 1, while the common-emitter current gain is larger. The numerical value.

In NPN bipolar transistor, the area of the collector junction is larger than the emitter junction. In addition, the emitter has a relatively high doping concentration.

Under normal circumstances, several regions of bipolar transistors are asymmetric in physical properties and geometric dimensions. Assuming that the transistor connected in the circuit is located in the forward amplifier region, if the connection of the transistor collector and emitter in the circuit is interchanged at this time, the transistor will leave the forward amplifier region and enter the reverse working area.

The internal structure of the transistor determines that it is suitable for working in the forward amplifier region, so the common-base current gain and common-emitter current gain in the reverse working region are much smaller than that in the forward amplifier region.

This functional asymmetry is basically due to the different doping levels of the emitter and collector. Therefore, in an NPN transistor, although the collector and emitter are both N-type doped, the electrical properties and functions of the two cannot be interchanged at all.

The emitter region has the highest degree of doping, the collector region is second, and the base region has the lowest degree of doping. In addition, the physical dimensions of the three regions are also different. The base region is very thin, and the collector area is larger than the emitter area. Because the bipolar transistor has such a material structure, it can provide a reverse bias for the collector junction, but the premise is that the reverse bias cannot be too large or the transistor will be damaged. The purpose of heavily doping the emitter is to increase the efficiency of injecting electrons from the emitter into the base region, so as to achieve the highest possible current gain.

In the common emitter connection of bipolar transistors, small changes in the voltage applied to the base and emitter will cause significant changes in the current between the emitter and the collector. Using this property, you can amplify the input current or voltage.

Regarding the base of the bipolar transistor as the input and the collector as the output, the two-port network can be analyzed by using the Thevenin theorem. Using the equivalence principle, a bipolar transistor can be regarded as a voltage-controlled current source or as a current-controlled voltage source.

1. NPN Type

The NPN transistor is one of two types of bipolar transistors. It consists of two layers of N-type doped regions and a layer of P-type doped semiconductor (base) between them. The tiny current input to the base will be amplified, generating a larger collector-emitter current.

When the base voltage of the NPN transistor is higher than the emitter voltage, and the collector voltage is higher than the base voltage, the transistor is in a forward amplifier state. In this state, there is a current between the collector and emitter of the transistor. The amplified current is the result of electrons injected by the emitter into the base region (minority carriers in the base region) and drifted to the collector under the push of an electric field. Since electron mobility is higher than hole mobility, most bipolar transistors in use today are of the NPN type.

The electrical symbol of the NPN bipolar transistor is shown on the right, and the arrow between the base and the emitter points to the emitter.

Figure 2. a) NPN Bipolar Transistor Symbol b) PNP Bipolar Transistor Symbol

2. PNP Type

Another type the PNP bipolar transistor consists of two layers of P-type doped regions and a layer of N-type doped semiconductors in between. The tiny current flowing through the base can be amplified at the emitter end. In other words, when the base voltage of the PNP transistor is lower than that of the emitter, the collector voltage is lower than the base voltage, and the transistor is in the forward amplifier region.

In the bipolar transistor symbol, the arrow between the base and the emitter points to the direction of the current. Contrary to the NPN type, the arrow of the PNP type transistor points from the emitter to the base.

3. Heterojunction

Heterojunction bipolar transistor is an improved bipolar transistor, which has the capability of high-speed operation. Studies have found that this transistor can handle ultra-high frequency signals with frequencies up to several hundred GHz, so it is suitable for applications that require harsh working speeds such as RF power amplifiers and laser drivers.

The heterojunction is a type of PN junction. The two ends of this junction are made of different semiconductor materials. In this type of bipolar transistor, the emitter junction usually adopts a heterojunction structure, that is, a wide-band-gap material is used in the emitter region, and a narrow-band-gap material is used in the base region. The common heterojunction uses GaAs to make the base region and AlxGa1-xAs to make the emitter region. With such a heterojunction structure, the injection efficiency of the bipolar transistor can be improved, and the current gain can also be improved by several orders of magnitude.

The doping concentration of the base region of a bipolar transistor with a heterojunction can be greatly increased so that the resistance of the base electrode and the width of the base region can be reduced. In a traditional bipolar transistor, that is, a homojunction transistor, the carrier injection efficiency from the emitter to the base is mainly determined by the doping ratio of the emitter and the base. In this case, in order to obtain higher injection efficiency, the base region must be lightly doped, which inevitably increases the base resistance.

In the base region, the composition of the semiconductor material is unevenly distributed, resulting in a gradually changing bandgap of the base region. This slowly varying forbidden bandwidth can provide an internal electric field for minority carriers to accelerate them through the base region. This drift motion will have a synergistic effect with the diffusion motion to reduce the transit time of electrons through the base region, thereby improving the high-frequency performance of the bipolar transistor.

Although many different semiconductors can be used to construct heterojunction transistors, silicon-germanium heterojunction transistors and aluminum-gallium arsenide heterojunction transistors are more commonly used. The process of manufacturing heterojunction transistors is crystal epitaxy, such as metalorganic vapor phase epitaxy (MOCVD) and molecular beam epitaxy.

IV Parameters

1. Collector Dissipation Power

The maximum collector dissipation power of a bipolar transistor is the maximum power that the device can work normally under certain temperature and heat dissipation conditions. Under the same conditions, if the actual power is greater than this value, the temperature of the transistor will exceed the maximum allowable value, degrading the device performance and even causing physical damage.

2. Current and Voltage

When the collector current increases to a certain value, although the bipolar transistor will not be damaged, the current gain will be significantly reduced. In order for the transistor to work normally as designed, it is necessary to limit the value of the collector current. In addition, because bipolar transistors have two PN junctions, their reverse bias voltage cannot be too large to prevent the reverse breakdown of the PN junction. The bipolar junction transistor datasheet will list these parameters in detail.

When the reverse bias voltage of the collector of the power bipolar transistor exceeds a certain value and the current flowing through the transistor exceeds a certain allowable range, making the transistor power greater than the critical power of the secondary breakdown, a kind of dangerous phenomenon of "second breakdown" will be produced. In this case, the current beyond the design range will cause local temperature imbalance in different areas inside the device, and the temperature in some areas is higher than in other areas.

Because doped silicon has a negative temperature coefficient, its conductivity is stronger when it is at a higher temperature. In this way, the hotter part can conduct more current, and this part of the current will generate additional heat, causing the local temperature to exceed the normal value, so that the device can not work normally.

The secondary breakdown is a kind of thermal runaway. Once the temperature rises, the conductance will further increase, causing a vicious circle and ultimately severely destroying the structure of the transistor. The entire secondary breakdown process can be completed in milliseconds or microseconds.

If the emitter junction of a bipolar transistor provides a reverse bias that exceeds the allowable range and does not limit the current flowing through the transistor, an avalanche breakdown will occur in the emitter junction, damaging the device.

3. Temperature Drift

As an analog device, all parameters of bipolar transistors are affected by temperature to varying degrees, especially the current gain. According to research, every 1 degree Celsius increase in temperature, the current gain will increase by approximately 0.5% to 1%.

4. Radiation Resistance

Bipolar transistors are more sensitive to ionizing radiation. If the transistor is placed in an environment of ionizing radiation, the device will be damaged by the radiation. The damage occurs because radiation will produce defects in the base area, which will form recombination centers in the energy band. This will result in a shorter lifetime of minority carriers that function in the device, which in turn will gradually reduce the performance of the transistor.

NPN-type bipolar transistors have a larger effective recombination area of carriers in a radiation environment, and the negative impact is more significant than that of PNP-type transistors. In some special applications, such as electronic control systems in nuclear reactors or spacecraft, special measures must be used to mitigate the negative effects of ionizing radiation.

V Working Region

According to the bias state of the three terminals of the transistor, several different working regions of the bipolar transistor can be defined. In NPN semiconductors (note: the voltage profiles of PNP transistors and NPN transistors are exactly the opposite), according to the bias of the emitter junction and the collector junction, the working region can be divided into:

1. Bipolar Transistor Amplifier Region

(1) Forward Amplifier Region

When the emitter junction is forward biased and the collector junction is reverse biased, the transistor works in the amplifier region. The design goal of most bipolar transistors is to obtain the maximum common-emitter current gain bf in the forward amplifier region. When the transistor works in this region, the collector-emitter current and the base current are approximately linear. Due to the current gain, when the base current is slightly disturbed, the collector-emitter current will change significantly.

(2) Reverse Amplifier Region 

If the above-mentioned bias voltages of the emitter and collector of the transistor in the forward amplifier region are interchanged, the bipolar transistor will work in the reverse amplifier region. In this mode of operation, the function of the emitter and collector regions are exactly opposite to that in the forward amplifier region. However, since the doping concentration of the transistor collector is lower than that of the emitter, the effect produced by the reverse amplifier region is not the same as the forward amplifier region.

The design goal of most bipolar transistors is to get the maximum forward amplifier current gain as much as possible. Therefore, the current gain in the reverse amplifier region will be smaller than that in the forward amplifier region. In fact, this working mode is hardly adopted, but in order to prevent device damage or other dangers caused by the wrong connection, it must be considered in the design. In addition, some types of bipolar logic devices also consider the reverse amplifier region.

Figure 3. BJT Forward Reverse Cut-off and Saturation

2. Saturation Region

When the two PN junctions in the bipolar transistor are both forward biased, the transistor will be in the saturation region. At this time, the current from the emitter to the collector of the transistor reaches the maximum value. Even if the base current is increased, the output current will not increase anymore. The saturation region can be used to indicate a high level of logic devices.

3. Cut-off Region

If the bias of the two PN junctions of the bipolar transistor is exactly opposite to that in the saturation region, then the transistor will be in the cut-off region. In this mode of operation, the output current is very small (less than 1 microampere for low-power silicon transistors, and less than even microampere for germanium transistors), which can be used to represent low levels in digital logic.

4. Avalanche Breakdown

When the reverse bias applied to the collector junction exceeds the range that the collector junction can withstand, the PN junction will be broken down. If the current is large enough, the device will be damaged.

In addition, when we analyze and design bipolar transistor circuits, it should be noted that the maximum collector dissipation power Pcm of the bipolar transistor cannot be exceeded. If the working power of the transistor is less than this value, the collection of these working states is called the safe working area. If the working power of the transistor exceeds this limit, the temperature of the device will exceed the normal range, and the performance of the device will change significantly and even cause damage.

The allowable junction temperature of silicon transistors is between 150 degrees Celsius and 200 degrees Celsius. The maximum allowable power dissipation can be increased by reducing internal thermal resistance, using heat sinks, and introducing measures like air cooling, water cooling, and oil cooling.

In fact, there is no absolute limit between the above-mentioned working regions. Within the range of small voltage changes (less than a few hundred millivolts), there may be a certain overlap between the different regions.